The invention relates to armature design for low cog high efficiency DC motors.
DC motors, particularly of the brushless variety, are used in a number of applications requiring precisely constant rotation, for example, in magnetic tape or disk drives. A typical brushless DC motor has an inner rotor with a ring of alternately permanently magnetized segments inside a stationary circular array of laminar pole pieces carrying a plurality of electrical coils usually wound about radial axes, and electrically connected in a three-phase delta circuit. Conversely a DC motor with a commutated rotor usually has the permanent magnets arranged in a stationary ring outside the coil carrying poles forming the rotor. In any case, there are two distinct parts to this type of DC motor: an electromagnetic part and a permanent magnet part. Various motor designs may employ either part as the rotor. The term "armature" has come to mean that part of an electric rotating machine that includes the main current-carrying winding in which the electromotive force produced by magnetic flux rotation is induced. An armature may be rotating or stationary. On the other hand, the term "field" is used in contrast to armature to refer to a permanent magnet or electromagnet producing a constant strength magnetic field in an electric rotating machine. The field may be on the stator or the rotor. The terms armature and field as used herein are thus employed to distinguish between two relatively rotatable members, one of which (the field) is predominantly characterized by a constant strength magnetic field and the other of which (the armature) is characterized by the presence of electrical coils which, in the case of a motor, are fed current of varying magnitude to produce a varying magnetic field.
To achieve high efficiency, compact DC motors require armature poles with saliencies which project toward the field magnets so that the surfaces of the saliencies and field magnets are very close. This juxtaposition makes the motor more efficient at converting electrical energy to torque. However, it also makes the motor more vulnerable to cogging, a mechanical force which manifests itself as a resultant torque through which the rotor seeks to hold relatively stable positions of minimum reluctance. Cogging is most prevalent in the absence of active energization of the armature, particularly during deceleration.
Another problem area with small high efficiency DC motors is the design and manufacture of the armature windings and cores. Conventional designs complicate manufacture, unduly limit the space for the coils, and exhibit undesirably high inductance during switching of phases.